Process for depolymerizing polystyrene



Patenterl Sept. 26, 1944 rnocnss FOR nnronmnmzmc POLYSTYBENE Joseph 0.Frank and James L. Amos, Midland, and Albert F. Straubel, Auburn, Michassignors to The Dow Chemical Company, Midland, Mich., acorporation ofMichigan No Drawing. Application September 17, 1941,

Serial No, 411,150

i 2Claims. (01.260-669) This invention relates to a method for thedepolymerization of polystyrene to produce lower styrene polymers andmonomeric styrene.

In the commercial preparation and use of solid resinous polystyrene, e.g., for the manufacture of molded articles, axproblem of economicimportance is the utilization of trimmings, offgrade batches, and othertypes of scrap material. One method that has been suggested for theprofitable utilization of such material is its depolymerization to thelower polymeric forms by heating. Such depolymerization of polystyrenehas also been shown to be of importance as one step in a method for therecovery of styrene from drip oil, cracked petroleum fractions, and thelike, where the styrene is present in low concentrations'and usuallyassociated with other compounds boiling over the same range as styrene.Thus, a styrene-containing fraction obtained from such sources may betreated to polymerize which may then be separated from the fraction anddepolymerized by direct heating to styrene and lower polymers which maythen be purified or utilized in known manner.

The pyrolysis of polystyrene has not heretofore been commerciallyfeasible due principally to the extremely low heat conductivity ofpolystyrene and the consequent difficulty of heating the main body ofthe material without overheating the outer layers. It has been shownthat when the depolymerization is carried out by heating the polystyreneunder vacuum to remove the styrene and lower polymers rapidly from thehot zone, a longer heating period is required and little, if any,decrease in the amount of by-products solid state. Liquid polymericstyrene may be run into the depolymerization vessel and superheatedsteam blown through the liquid, or it may be treated withsteamcounter-currently in a tower. The monomeric styrene vapors together,usually, with considerable amounts of the lower polymeric forms ofstyrene 'and the steam may be con- 7 densed and collected in thereceiver, or they may be fractionally condensed to separate the lowerpolymers from the styrene.

The depolymerization is usually carried out at atmospheric pressure,although higher or lower pressures may be used if desired. Steam at atemperature between 250 and 600 C., preferably between 300 and 450 C.,is used in the process. Although the amount of steam used relative tothe amount of polystyrene depends upon several factors, e. g., the rateof flow of the steam, the degree of contact between the polystyrene andthe steam/and the particular temperthe styrene to a high molecularweight product ,ature used, it is usually from five to twenty-five timesthe weightof the polystyrene, and may be more or less than this amount.

The oily layer in the receiver may be separated from the aqueous layerand treated in any suitformed is obtained. When the pyrolysis is carriedout by such previously known methods there are formed, in addition tostyrene and its lower polymeric forms, appreciable quantities ofundesirable hydrocarbons, e. g., 1,3-diphenylpropane,1,3,5-triphenylpentane, 1,3,5-triphenylbenzene and toluene, togetherwith ten to twenty per cent of tarry residue.

We have now found that polystyrene may be ing the superheated steam withit. Solid polystryene may be broken up and fed continuously into thedepolymerization vessel and be subjected to the action of steam in thesolid orsemipolymerization experiment.

able manner, e. g., by steam distilling, to separate the monomericstyrene. Lower polymeric forms of-styrene may be fed back into theprocess to produce additional monomeric styrene, if desired, or the oilremaining after steam distilling the monomeric styrene may befractionally distilled to isolate fractions rich in a single lowerpolymer oi styrene, e. g., the dimer, the trimer, or the -tetramer,which may be collected as final products.

The following examples will serve to illustrate the principles of theinvention, but are not to be construed as limiting its scope:

' Example 1 4390' grams of polystyrene was placed in an iron reactorfitted with a condenser and receiver. 60,780 grams of steam at atemperature of 350 C. and at atmospheric pressure-was passed through thereactor. 4321 grams of oily material were separated from the water inthe receiver. This was a recovery of 98.4 per cent based on thepolystyrene used. The oily layer from the receiver was iractionallydistilled to recover monomeric styrene. There was thus obtained 2250grams of the latter compound of 98.4 per cent purity or a 51.4 per centyield on the basis of the polystyrene started with. The residue from thefractional. distillation was fed back into a. subsequent de- Example 2Other modes of applying the prinoiple of our invention may be employedinstead of those explained, change being made as regards the methodherein disclosed. provided the step or steps stated by any of thefollowing claims or the ployed.

We therefore particularly point out and dis tinctly claim asauxinvention: r

1. The method which comprises heating polystyrene to a depolymerizingtemperaturebetween 250 and 600 C. by passing superheated steam of atleast as high a. temperature into contact there- 5 with end condensingthe styrene which is evolved together with the steam.

2. In a method of depolymerizing olystyrene. the steps of heating thelatter to a. depoiymerizlng temperature between 300" and 450 C. byposing 10 steam which has been superheated to a temperature higher thanthe depolymerizing temperature into intimate contact with thepolystyrene, condensing the evolved depolymerization product:andfrsetionallydisflllingthelattertoreeover 15 therefrom a. fraction ofstyrene and mother fraction of dimeric styrene.

equivalent of such stated step or steps be em- 7 JOSH'H (LIT-ARK.JAMELAMOB.

m ALBERT r. STRAUBEL.

